British patent specification No. 2,037,731 discloses a reactor provided with an agitator which is rotated around a horizontal shaft and in which the recycling of the solids settled on the bottom of the vessel from the outlet end of the reactor to the inlet end is performed by means of a pipe rotating together with the agitator and forming an angle with the shaft. The disadvantage of this known solution is that the amount of the recycled solids cannot be adjusted. The return pipe may be clogged and then the entire reactor has to be opened and emptied.